The invention relates to a hybrid coupler having four ports and capable of coupling radio frequency signals having of a certain frequency from at least one port to at least one other port. The invention further relates to an amplifying circuit comprising such hybrid couplers, and a portable radio communications device comprising such an amplifying circuit. The invention also relates to a method of amplifying radio frequency signals.
Radio transmitters as they are used in e.g. portable radio communications devices often have a power amplifier (PA) separated from the rest of the radio circuit, and the power amplifier is often connected to an antenna through an isolator which is provided to compensate for an impedance mismatch by the output load (i.e. the antenna) presented to the output of the power amplifier. Without the isolator the mismatch would result in an unsatisfactory value of the VSWR (Voltage Standing Wave Ratio).
Especially in portable devices there is a demand for miniaturization of the circuits, and one way of obtaining this would be to integrate the power amplifier on the same chip as the rest of the radio circuit and/or to avoid the isolator.
However, the integration of the power amplifier together with the more sensitive parts of the radio circuit will typically result in increased distortion because the power amplifier produces ripple on the supply voltage to the more sensitive circuits. In addition, this problem increases because of the tendency to use lower and lower supply voltages in such circuits. Unchanged output power from a lower voltage means a higher current and thus higher ripple on the supply voltage.
The ripple can be reduced by combining multiple transistors or other types of amplifiers in the power amplifier, provided the transistors/amplifiers are not conducting in phase. One state of the art solution is to use a differential amplifier. In this solution the current through the transistors from the supply voltage to ground is close to being constant as long as the amplifier is running in its linear region. However, such power amplifiers are generally driven so strongly that overloading occurs, i.e. they are run in their non linear region in which ripple (pulses) is still produced on the supply voltage. The benefit of the differential amplifier compared to a one-transistor amplifier is that the amplitude of the ripple is halved and the frequency is doubled, but the result is still not acceptable.
Regarding the removal of the isolator, the power amplifier can be re-biased to keep it in its linear range even with the load mismatch, but this prevents the amplifiers from being driven sufficiently strongly, and it also requires a relatively complicated regulating circuit.
Another solution utilizing the combination of multiple transistors is to use amplifying circuits with hybrid couplers. Such amplifiers are known to be less sensitive to output load mismatch, or at least they can be modified to be so. An example of this is disclosed in U.S. Pat. No. 4,656,434. Thus the isolator can be avoided. In this type of amplifier there is a 90xc2x0 phase shift between the conducting periods of the two transistors. Similar to the differential amplifier the amplitude of the ripple on the supply voltage is substantially halved. There will typically be frequency components of the operating frequency and of twice the operating frequency. Again, this is an improvement compared to the one-transistor amplifier, but it is still not sufficient.
Therefore, it is an object of the invention to provide a hybrid coupler allowing an amplifying circuit to be produced which has sufficiently low ripple on the supply voltage to be integrated together with more sensitive radio circuits, and which is also insensitive to load mismatch such that an isolator can be avoided.
According to the invention the object is achieved in that the hybrid coupler is implemented as a differential coupler arranged to couple differential radio frequency signals.
A differential hybrid coupler allows the output current to be shared between four transistors or amplifiers, thus reducing the amplitude of the ripple to a much lower level than that of the one-transistor amplifier. Further, the conducting periods of the four transistors are equally spaced with a 90xc2x0 phase shift between them, and thus the frequency of the ripple is four times the operating frequency of the circuit, which makes it much easier to filter out the ripple in other parts of the circuit.
In one embodiment of the invention the hybrid coupler is implemented in a stripline technology, and in another embodiment it is implemented in a microstrip technology. Thus the hybrids can easily be integrated together with other circuits in one of these technologies.
In an expedient embodiment the hybrid coupler is a 3 dB coupler, such that power of the frequency supplied to one port is split substantially equally between two other ports, while the remaining port is substantially isolated from the other ports. In this way it is ensured that the output current is shared equally between the four transistors or amplifiers so that the amplitude of the ripple is reduced to one quarter of that of the one-transistor amplifier.
The hybrid coupler may be arranged to split the power between the two other ports in such a way that the signals provided at these ports are in phase with each other. This allows a simple type of hybrid to be used, but the electrical lengths of the connections between the outputs of the hybrid at the input side of the amplifying circuit and the two amplifiers must differ by a quarter of a wavelength for the signals of the operating frequency in order to ensure that the two amplifiers still conduct with a phase shift of 90xc2x0 between them.
Alternatively, the hybrid coupler may be arranged to split the power between the two other ports in such a way that the signals provided at these ports are in quadrature to each other. This allows connections with equal electrical lengths to be used from the outputs to the amplifiers, because the output signals already have a 90xc2x0 phase difference. In an expedient embodiment this hybrid coupler is a line coupled hybrid.
As mentioned, the invention further relates to an amplifying circuit for radio frequency signals having a certain frequency and thus a certain wavelength. This circuit comprises at least a first hybrid coupler having an input port to which radio frequency signals can be applied, an isolated port, a first output port, and a second output port, and being arranged for dividing a signal applied to the input port into a first signal component to the first output port and a second signal component to the second output port; a first amplifier having an input port and an output port, said input port being connected to the first output port of the first hybrid coupler; a second amplifier having an input port and an output port, said input port being connected to the second output port of the first hybrid coupler; and a second hybrid coupler having a first input port connected to the output port of the first amplifier, a second input port connected to the output port of the second amplifier, an isolated port, and an output port connectable to an output load impedance, and being arranged for combining signals applied to the first input port and the second input port to the output port, said first and second hybrid couplers and said first and second amplifiers providing a first and a second path for radio frequency signals from the input port of the first hybrid coupler to the output port of the second hybrid coupler, said first path comprising the first amplifier and said second path comprising the second amplifier; and wherein the total electrical lengths of the two paths are substantially identical, and the electrical length from the input port of the first hybrid coupler to each of the input ports of the first and second amplifiers differs by a quarter of a wavelength for said radio frequency signals.
When the hybrid couplers are implemented as differential couplers arranged to couple differential radio frequency signals, and the amplifiers are differential amplifiers, an amplifying circuit is provided which has sufficiently low ripple on the supply voltage to be integrated together with more sensitive radio circuits, and which is also insensitive to load mismatch such that an isolator can be avoided.
A differential hybrid coupler allows the output current to be shared between four transistors or amplifiers, thus reducing the amplitude of the ripple to a much lower level than that of a one-transistor amplifier. Further, the conducting periods of the four transistors are equally spaced with a 90xc2x0 phase shift between them, and thus the frequency of the ripple is four times the operating frequency of the circuit, which makes it much easier to filter out the ripple in other parts of the circuit.
In one embodiment of the invention the first and second hybrid couplers are implemented in a stripline technology, and in another embodiment they are implemented in a microstrip technology. Thus the hybrids and the amplifying circuit can easily be integrated together with other circuits in one of these technologies.
In an expedient embodiment the first and second hybrid couplers are 3 dB couplers. In this way it is ensured that the output current is shared equally between the four transistors or amplifiers so that the amplitude of the ripple is reduced to one quarter of that of the one-transistor amplifier.
The first and second hybrid couplers may be in-phase couplers, such that said first and second signal components on the output ports of the first hybrid coupler are in phase with each other, and signals in phase with each other applied to the two input ports of the second hybrid coupler are combined to one signal at its output port. This allows a simple type of hybrid to be used, but the electrical lengths of the connections between the outputs of the hybrid at the input side of the amplifying circuit and the two amplifiers, and those between the two amplifiers and the inputs of the hybrid at the output side of the amplifying circuit, must differ by a quarter of a wavelength for the signals of the operating frequency in order to ensure that the two amplifiers still conduct with a phase shift of 90xc2x0 between them.
Alternatively, the first and second hybrid couplers may be quadrature couplers, such that the first and second signal components on the output ports of the first hybrid coupler are in quadrature to each other, and signals in quadrature to each other applied to the two input ports of the second hybrid coupler are combined to one signal at its output port.
This allows connections with equal electrical lengths to be used from the outputs of the hybrid at the input side of the amplifying circuit to the amplifiers, and from the amplifiers to the inputs of the hybrid at the output side of the amplifying circuit, because the output signals already have a 90xc2x0 phase difference. In an expedient embodiment the first and second hybrid couplers are line-coupled hybrids.
As mentioned, the invention further relates to a portable radio communications device comprising an amplifying circuit as described above. Due to the above-mentioned advantages such a device can be further miniaturized, because the power amplifier can be integrated together with other parts of the radio circuit, and the isolator may be avoided. In an expedient embodiment the device is a mobile telephone.
As mentioned, the invention further relates to a method of amplifying radio frequency signals having a certain frequency and thus a certain wavelength. The method comprises the steps of applying radio frequency signals to an input port of a first hybrid coupler; dividing the signals applied to the input port into a first signal component to a first output port of the first hybrid coupler and a second signal component to a second output port of the first hybrid coupler; amplifying said first signal component in a first amplifier having an input port and an output port, said input port being connected to the first output port of the first hybrid coupler; amplifying said second signal component in a second amplifier having an input port and an output port, said input port being connected to the second output port of the first hybrid coupler; coupling the amplified first signal component from the output port of the first amplifier to a first input port of a second hybrid coupler and the amplified second signal component from the output port of the second amplifier to a first input port of the second hybrid coupler; combining in the second hybrid coupler the signals applied to the input ports thereof to an output signal on the output port of the second hybrid coupler; and coupling said output signal to an output load impedance; wherein the total electrical lengths of the paths of the two signal components from the input port of the first hybrid coupler to the output port of the second hybrid coupler are substantially identical, and the electrical length from the input port of the first hybrid coupler to each of the input ports of the first and second amplifiers differs by a quarter of a wavelength for said radio frequency signals.
When the radio frequency signals are applied, coupled and amplified as differential signals from the input port of the first hybrid coupler to the output port of the second hybrid coupler, an amplifying method is provided which has sufficiently low ripple on the supply voltage to allow a corresponding circuit to be integrated together with more sensitive radio circuits, and which is also insensitive to load mismatch such that an isolator can be avoided.
Differential amplification allows the output current to be shared between four transistors or amplifiers, thus reducing the amplitude of the ripple to a much lower level than that of a one-transistor amplifier. Further, the conducting periods of the four transistors are equally spaced with a 90xc2x0 phase shift between them, and thus the frequency of the ripple is four times the operating frequency of the circuit, which makes it much easier to filter out the ripple in other parts of the circuit.